Top Quark Physicsat the Large Hadron Collider
David Schaich (Amherst College)
Šarka Todorova (Tufts University)Krzysztof Sliwa (Tufts University)
Five College SymposiumUniversity of Massachusetts
1 October 2005
UM-CERN NSF-REU Program
Outline● CERN
● The Large Hadron Collider (LHC)
● Top Physics – Production and Decay
● Top Mass in the Dilepton Channel
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CERNThe European Organization for Nuclear Research
The world's largest particle physics laboratory and research center, located outside Geneva Switzerland on the SwissFrench border
Founded in 1954 as a multinational collaboration
Now includes 20 European Member States
USA, Russia, EU, UNESCO, Japan, Turkey and Israel have observer status
Flagship project: the LHC
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Large Hadron Collider (LHC)
14-TeV proton-proton accelerator
Currently under construction:
First beams, 2007
First physics runs 2008
A hadronic 'discovery' accelerator
Will search for Higgs, supersymmetry, quark-gluon plasmas, CP-violation, physics beyond standard model
Also important for top quark physics
A “Top Factory”
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Top QuarksHeaviest matter particle, by far
Detected only ten years ago
Very heavy – hard to produce
Much still measured with only little precision
MassSpinPolarizationDecaysBare quarks?
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Top Quarks Beyond the SM
Can also be used to probe physics beyond the Standard Model
Top mass constrains Higgs mass (light Higgs favored by top mass data)
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Top Quark ProductionDominant production mechanism is topantitop pair production through either
Tevatron: 85% 15%LHC: 5% 95%
Quark-antiquark annihilation
Gluon-gluonfusion
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Top Quark Decay and Detection
Tops decay very quickly into a b quark and W boson. The W can then decay either leptonically or hadronically.This gives three channels of topantitop decay:
Dilepton channel: or e only: 5%
Lepton + jets channel: or e only: 30%
Alljets channel:44%, messy
Which is best for measurements?Leptons easy to measureJets less soNeutrinos not at all
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Top Mass in the Dilepton Channel
Have very accurate lepton measurements, but nothing at all for the neutrinos.
Task is to determine eight unknowns (four-momentum components of the two neutrinos) from eight kinematical equations.
Use a geometrical approach for each quarkConstant E
t gives a circle in momentum space
Varying Et produces a paraboloid
Points of constant Mt lie on a plane section of the paraboloid
– i.e., an ellipseProject the ellipse onto the transverse momentum plane
Ellipses for top and antitop should match!
See Dalitz, R.H.& Goldstein, G.R. 1992 Ph ys. Rev . D45, 1531-1543
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The Project
My project was to take code from the CDF experiment at Fermilab that measures the mass of the top quark using the dilepton channel approach just described and adapt it to the ATLAS experiment.
Then I would test the code with samples of increasingly complex and realistic data, in preparation for real data from ATLAS.
The work is still in progress after many complications, most rather technical and uninteresting.
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An Interesting Complication
Top mass:152 GeV(± ~10 GeV)
Code gives very low results for real Fermilab data (through 2004)
'Official' mass:174±3.4 GeV
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Acknowledgments
CERN
University of MichiganProf. Homer NealProf. Jean Krish
National Science Foundation
Ford Motor Company
Tufts UniversityDr. Šarka TodorovaProf. Krzysztof Sliwa
